1. Field of the Invention
The present invention relates to a voltage controlled oscillator (VCXO: Voltage Controlled Crystal Oscillator), and more particularly relates to a voltage controlled oscillator enabling a large variable amount of oscillatory frequency.
2. Description of the Related Art
[Prior Art]
In recent years, a large variable amount of frequency has been required for a voltage controlled oscillator.
In a conventional voltage controlled oscillator, a variable amount of oscillatory frequency is secured by applying a control voltage to a variable-capacitance diode connected in series with a crystal resonator and by further connecting an expansion coil in series between the crystal resonator and the variable-capacitance diode.
As another frequency adjusting method for oscillator, a conventional voltage controlled oscillator typically uses a method of replacing a fixed capacitor to change a load capacitance.
More specifically, capacitors connected in parallel are inserted between a crystal resonator and a variable-capacitance diode connected in series with the crystal resonator, and one of the capacitors connected in parallel is replaced so as to increase a load capacitance of a circuit, thus adjusting the frequency.
Note that a control voltage applied to the variable-capacitance diode is varied so as to change a capacitance of the variable-capacitance diode, whereby a frequency of the voltage controlled oscillator can be changed.
[Related Art]
Related art includes: Japanese Patent Application Laid-Open No. 2001-16039 “Variable frequency oscillation circuit” (Applicant: Kinseki Corporation/Patent Document 1),
Japanese Patent Application Laid-Open No. H07-15238 “Voltage controlled oscillator” (Applicant: Mitsubishi Materials Corporation/Patent Document 2), and Japanese Patent Application Laid-Open No. 2000-183650 “Voltage oscillator” (Applicant: Toyo Communication Equip Co Ltd/Patent Document 3).
Patent Document 1 discloses a variable frequency oscillation circuit, and FIG. 1 illustrates a crystal vibrating reed and a variable-capacitance diode connected with an oscillation circuit are connected in series with a circuit including an inductance and a capacitor connected in parallel.
As compared with a conventional circuit of FIG. 5 in Patent Document 1, the capacitor and the inductance are arranged in parallel, so that a frequency variable amount can be increased.
Patent Document 2 discloses a voltage controlled oscillator in which both ends of a piezoelectric vibrating reed are connected via a circuit including a parallel circuit of a variable capacitor and a coil. As compared with a conventional circuit of FIG. 4 in Patent Document 2, the circuit of FIG. 1 in Patent Document 2 can increase a frequency variable amount.
Patent Document 3 discloses a voltage oscillator in which one end of a serially-connected circuit including a first resistor, a digital variable resistor IC and a second resistor is connected with a power source of a Colpitts crystal oscillator and the other end is grounded, and an output voltage of the digital variable resistor IC is connected with a cathode of a variable-capacitance diode.
In Patent Document 3, the digital variable resistor IC is regulated so that a capacitance of the diode is changed and a frequency is adjusted, thus making it easy to adjust the frequency as compared with the case of using a trimmer capacitor or the like.